Plasminogen activation is the central event in fibrinolysis, which is of importance to the strategy of short term treatments of acute thrombolytic disorders. Plasminogen activation also plays critical roles in cell migration related to tumor growth and metastasis, Alzheimer's disease and related cerebral hemorrhage and some pathogenic invasions. Streptokinase, a bacterial protein, is a plasminogen activator widely used in the clinical treatment of myocardial infarction and other clotting disorders. Unlike tissue plasminogen activator and urokinase, streptokinase is not a protease. Streptokinase and plasminogen form a non-covalent complex, which is proteolytically active and converts other plasminogen molecules to plasmin leading to fibrinolysis. Previously, we determined the crystal structures of the catalytic domains of plasminogen and plasmin and the structure of streptokinase. While these structures have provided significant insight into the plasminogen activation and interactions between streptokinase and plasminogen, they also raise new questions about the detailed mechanisms by which the plasminogen:streptokinase complex activates other plasminogen molecules and about the regulation of plasminogen activation by a variety of effectors. The current application proposes to address questions on the mechanism of nonproteolytic activation of plasminogen in the plasminogen:streptokinase complex, the interaction of the substrate plasminogen with the activator complex and interactions between plasminogen activation and some of its physiological/pathological effectors using mutagenesis and x-ray crystallographic approaches.